This invention relates to methods for the processing and purification of tantalum containing ores and compounds. In particular, it relates to methods of producing a high purity tantalum compound from standard grade tantalum oxides, hydroxides and hydrated ammonium tantalates.
High purity tantalum metal and tantalum pentoxide have become increasingly important to the electronics industry in the preparation of advanced electronic materials used in the manufacture of devices such as surface acoustic wave filters, pyroelectric infrared sensors, optoelectronic devices and capacitors. Prior to 1957, niobium and transition metals were separated from tantalum by a fractional crystallization process known as the Marignac process which exploits the difference in solubility between K2TaF7 and K2NbOF5.H2O and other fluorometallates. However, tantalum pentoxide obtained by this process contains large impurities of Nb (1000-3000 ppm), and other elements such as Si (up to 3000 ppm), Ti (up to 100 ppm), and Fe (up to 2000 ppm). These levels are much greater than for example the total allowable transition metal impurity level (less than 1000 ppm) in the tantalum wire used in the manufacture of capacitors. In late 1950""s, modern solvent extraction and ion exchange processes supplanted the use of the Marignac process. Examples of liquid-liquid solvent extraction and ion exchange methods are disclosed in U.S. Pat. Nos. 3,117,833, 3,712,939, 4,673,554 and 4,446,115. In a solvent extraction process, ore concentrates containing at least 25 wt. % tantalum and niobium pentoxide are decomposed chemically in hydrofluoric acid media and the dissolved tantalum and niobium species are separated from the residue by filtration. The filtrate containing tantalum (as TaF72xe2x88x92) and niobium (as NbOF52xe2x88x92) in an HF/H2SO4 medium is brought into contact with an organic phase, usually methyl iso-butyl ketone (MIBK), which selectively absorbs tantalum and niobium species leaving impurities such as titanium, iron, and silicon in the aqueous phase. Niobium is separated from tantalum by back extraction with sulfuric acid. Finally, tantalum (TaF72xe2x88x92) is eluted from organic phase (MIBK) by an ammonium fluoride solution and converted into K2TaF7 by precipitation with KCl. Although Ta2O5 (or K2TaF7) made by solvent extraction method can be used for most applications, this procedure is costly, uses large quantities of HF which requires special handling, and requires the disposal of large quantities of solvents.
U.S. Pat. No. 5,635,146, which is incorporated herein by reference, describes an alternative method for the preparation of pure K2TaF7 and tantalum pentoxide. This method also requires the use of hydrofluoric acid albeit in lesser amounts than the above described methods.
U.S. Pat. No, 6,010,676 describes a direct dissolution method where an impure tantalum compound is slurried with sulfuric acid prior to dissolution in hydrofluoric acid.
It is an object of this invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide an economical method for purifying tantalum without the use of hydrofluoric acid.
It is a further object of the invention to provide a method to produce high purity K2TaF7 without utilizing solvent extraction or ion exchange techniques.
In accordance with the objects of this invention, there is provided a method of purifying a tantalum compound, comprising:
(a) combining a tantalum compound containing impurities with a fluoride containing compound and sulfuric acid to form a solution containing tantalum values and impurities; the tantalum compound being selected from a tantalum oxide, a tantalum hydroxide or an hydrated ammonium tantalum oxide, or a combination thereof; the fluoride containing compound being selected from ammonium bifluoride, NaF, KF, CaF2, or a combination thereof; and
(b) separating the tantalum values from the impurities to obtain a purified tantalum compound.
We discovered that technical grade hydrated ammonium tantalum oxide (HATO), (NH4)2xe2x88x92xHxTa2O6.nH2O), and related compounds such as tantalum hydroxide and tantalum oxide, which contain relatively high impurity levels, can be used to produce a highly pure tantalum compound such as K2TaF7 by a direct dissolution process. The method of this invention preferably uses ammonium bifluoride, NH4HF2, as the fluoride source in place of the hydrofluoric acid used in the conventional methods. This is particularly advantageous because ammonium bifluoride is about 70% less expensive (in terms of F values) and much safer and easier to handle and store than HF.
Generally, the method comprises dissolving HATO, tantalum hydroxide and/or tantalum oxide and a fluoride containing compound in sulfuric acid. The preferred fluoride containing compound is ammonium bifluoride. Other fluoride containing compounds include NaF, KF and CaF2, or a combination thereof which may be used separately or in combination with ammonium bifluoride. Some tantalum compounds such as tantalum pentoxide may require conversion to potassium tantalate prior to dissolution as described in U.S. Pat. No. 5,635,146. Once in solution, tantalum values are precipitated preferably as pure K2TaF7 by the addition of KCl or another water-soluble potassium compound such as K2CO3, KOH, KF, K2C2O4, or K2O. Crystals of the highly pure K2TaF7 product are separated by filtration, washed and dried at 110 xc2x0 C.